Work in progress is focused on elucidating the structure-function relationship of certain enzymes that use acyl-CoA as a substrate. The approach is based on affinity labeling methodology. Fatty acid synthetase from lactating rat mammary gland is under investigation using S-(4-bromo-2,3-dioxobutyl)-CoA as the affinity label. Carnitine acetyltransferase is under investigation using 12-(N-(4-azido-2-nitrophenyl))-aminodo-decanoyl-CoA as a photoaffinity label. An investigation of short chain acyl-CoA hydrolase is also underway.